This invention relates to muscle force and dispalcement transducers, and more particularly to a combination whereby measurements can be taken simultaneously of force and displacement within a very small area of the myocardium, and if desired an ECG can be taken in the same area.
Development of cardiac force transducers has been of growing interest for more than a decade. See "Auxotonic and Isometric Cardiac Force Transducers," by Eric O. Feigl, et al., Journal of Applied Physiology, Vol. 23, No. 4, October 1967, and references cited therein. There has also been an interest in measuring myocardian displacement. See U.S. Pat. No. 3,937,212 for a Miniature Muscle Displacement Transducer, issued on an invention made by Cyril Feldstein, et al. Now simultaneous measurements of myocardial force and displacement in a very small localized area parallel to subsurface fibers is becoming of interest to research scientists in the field of cardiology. For example, if both force and displacement can be measured simultaneously in a small area, the work being performed by the myocardium in that small area can be determined. It is also of interest simultaneously to provide an ECG as a reference for the heartbeat cycles at the same small area.
Work is directly determined by simultaneous measurement of force, F, and displacement, s, from the relationship between the force exerted on a body and the distance that the body moves in a direction of the force in producing work, W. That relationship is given by the equation EQU W=F.times.s cos .theta.
where .theta. is the angle between the direction of the force and the direction of the displacement. Orientation of myocardial fiber may be easily determined in a particular area by measuring force in all different directions to find that direction which yields the greatest measurememt. Once that orientation is determined, force and displacement may be measured simultaneously unmder a condition where cos .theta.=1 by placing all three tines in a line in the direction of fiber orientation. In other words, to measure work directly, it is desirable to measure force and displacement in the same direction as the fiber orientation. The product, W, will be a maximum when that measurement direction is parallel to the fiber orientation in the given localized area.
In the past, myocardial force or tension has been measured by a strain gauge sutured to the epicardial fibers. This has not been ideal because the measurements are too dependent on how tight and deep the sutures are made. Moreover, suturing is time consuming and traumatic to the myocardium, and sutured transducers are difficult to remove. In any case, the same suturing is hard to reproduce from one experiment to another, thus making it virtually impossible to compare data. As pointed out by Feigl, et al., supra, it is preferable to insert pins into the myocardium to measure the force acting on the pins, but still sutures are used to hold the transducer in place.
For measuring displacement, U.S. Pat. No. 3,937,212 cited above shows a transducer comprised of a curved beam of high elastic compliance connected at each end to a pin inserted into the myocardium. A piezoresistive element bonded to the curved beam then measures myocardial displacement between the pins. Because this transducer is intended to measure displacement only, it does not provide any force measurement in the same area as the displacement measurement. There is also the problem of holding the pins in. It would be feasible to provide a barb at the tip of each pin, as shown for the subminiature insertable force transducer disclosed in U.S. Pat. No. 3,905,536, but then the pins cannot be removed without tearing and cutting the myocardial tissue.